Segmentation in animals involves the longitudinal division of the body into serial sections, each typically having one pair of some or all of the various organ units. The terms metamerism or metameric segmentation are used only when organs of mesodermal origin are so arranged. Pseudometamerism refers to superficial segmentation and could be termed body annulation. This definition of metamerism applies to tapeworms, but this is a view many zoologists do not accept. A modern one now gaining favour is that cestodes are indeed metamerically segmented, although their metamerism is of a different type. 

    Metameric segmentation appears to have arisen independently three times: in the annelid-arthropod and chordate lines, and in the cestodes. The main theories concerning the origin of metamerism revolve around the questions as to whether it is the repetition of organs or mesodermal segmentation that is of primary importance, and whether there is a connection between the evolution of metamerism and of the coelom.





Pseudometamerism Theory 

    This theory proposes that the serial repetition of o~gans, or pseudo-segmentation, in some elongated turbellarians and nemerteans 'crystallised' into metameric segmentation, and was linked with coelom evolution. Metamerism was an accidental consequence of the serial arrangement in acoelomate animals of those structures associated with the coelomic pouches. Sub-division of the musculature was mandatory because in serpentine swimming the body could bend only in the spaces between coelomic compartments. However, all ribbon-like animals swim in this way whether segmented or not. 


Cyclomerism Theory 

 This theory is the corollary of the enterocoelous theory of mesoderm and coelom formation and implies the development of metaprot mes meric segmentation from the fundamentally radial organisation of met the actinians (Fig. 9). However, the theory depends on the acceptance of an ancestral coelenterate group with gastric pouches arranged in a linear and not a circular fashion, and also assumes that the original Bilateria were segmented and coelomate and that many groups now without these characters secondarily lost them. 


Corm Theory 

    This theory postulates that metameric segmentation resulted from incomplete separation following asexual reproduction during which a chain of zooids was formed. Such an event occurs in rhabdocoels and triclads (Fig. 10), in cestode proglottid formation, and in scyphozoan strobilae. The chief objection is that in platyhelminthes and scyphozoans the sequence of zooid formation is never serial with terminal fission; fission occurs always somewhere in the middle of the chain. In cestodes the proglottids are serially arranged but in a reverse order to the metameres of annelids and chordates; moreover, if cestodes are considered metamerically segmented, they are not eligible as ancestors to this condition. Another objection is that such reproduction is usually confined to sessile animals, unlikely ancestors to take advantage of the potentials of metamerism. 


Embryological Theory 

    This theory suggests that the formation of metameric segments originated mainly as an embryological accident. Originally, it suggested that mechanical stresses in the mesoderm during elongation of the embryo or larva resulted in mesoderm fragmentation, manifested in the adult as a meristic repetition of all mesodermal derivatives. An obstacle is the lack of segmentation in elongated nemerteans and tur bellarians.

    The theory has been partly revised by Berrill (1955) to account for chordate metamerism. He suggested that metamerism occurred in the tail muscles of an ascidian tadpole larva as an adaptation to swimming. But this still leaves unanswered the question of the origin of non-chordate segmentation.


Locomotory Theory 

    This theory postulates that undulatory, serpentine swimming movements completed the process of segmentation begun by the piecemeal repetition of organ systems; it is really an amalgamation of embryological and pseudometamerism theories. The major objection again is the lack of correlation between the ability to perform undulatory swimming motions and the possession of a segmented musculature.

    If most current ideas concerning metazoan evolution are accepted, then it is also necessary to accept the fact that metameric segmentation evolved at least three times. Each time it had a major advantage for the group in question: it evolved as an adaptation to burrowing in annelids, to swimming in chordates and to reproduction in cestodes. 

    Although many have supposed that metameric segmentation evolved in annelids to facilitate undulatory, serpentine swimming, Clark (1964) has shown that segmented musculature is not necessary for this. It arose, it seems, to facilitate burrowing and was used in conjunction with the coelom and particularly with a fluid-filled cavity divided by septa. The crucial step, therefore, was the evolution of septa impeding transmission through the body of locally generated fluid pressures. The coelom gave the animal a hydrostatic skeleton allowing the circular and longitudinal muscles to act against each other, but it was the septa and metameric segmentation together which allowed only part of the body to contract while other parts in the longitudinal axis relaxed, and enabled a strong peristaltic wave to be propagated down the body.

    Chordates are thought to have evolved from a free-swimming, ascidian, larva-like animal, and their metamerism is thought to have been associated with the evolution of the notochord as a continued response to the advantages of strong swimming. Berrill ( 19 55) postulated that originally the tail and notochord arose de novo as a sudden change caused by a developmental change in back cells. These grew out as a row of vacuolated cells, the notochord, forming a tail, and the mesodermal elements associated with them became arranged in metameric pattern. Metameric segmentation is certainly important in swimming in chordates as it allows powerful torsional forces to be applied to the relatively inflexible axial skeleton in order to produce powerful swimming movements.

    In cestodes the need to form many similar segments is purely reproductive; a reproductive package was formed which, once it had performed its function, was expendable. Such segments, it may be argued, are not additional to the body but are the body and so have to carry a complete series of organs.